title: Ribbon aromaticity in double-chain planar B(n)H2(2-) and Li2B(n)H2 nanoribbon clusters up to n = 22: lithiated boron dihydride analogues of polyenes.
abstract: We report an extensive density-functional theory and coupled-cluster CCSD(T) study on boron dihydride dianion clusters BnH2(2-) (n = 6-22) and their dilithiated Li2BnH2(0/-) salt complexes. Double-chain (DC) planar nanoribbon structures are confirmed as the global minima for the BnH2(2-) (n = 6-22) clusters. Charging proves to be an effective mechanism to stabilize and extend the DC planar nanostructures, capable of producing elongated boron nanoribbons with variable lengths between 4.3-17.0 Å. For the dilithiated salts, the DC planar nanoribbons are lowest in energy up to Li2B14H2 and represent true minima for all Li2BnH2(0/-) (n = 6-22) species. These boron nanostructures may be viewed as molecular zippers, in which two atomically-thin molecular wires are zipped together via delocalized bonds. Bonding analysis reveals the nature of π plus σ double conjugation in the lithiated DC nanoribbon Li2BnH2(0/-) (n up to 22) model clusters, which exhibit a 4n pattern in adiabatic detachment energies, ionization potentials, and second-order differences in total energies. Band structure analysis of the infinite DC boron nanoribbon structure also reveals that both π and σ electrons participate in electric conduction, much different from the monolayer boron α-sheet in which only π electrons act as carriers. A concept of "ribbon aromaticity" is proposed for this quasi-one-dimensional system, where regular π versus σ alternation of the delocalized electron clouds along the nanoribbons results in enhanced stability for a series of "magic" nanoribbon clusters. The total number of delocalized π and σ electrons for ribbon aromaticity collectively conforms to the (4n + 2) Hückel rule. Ribbon aromaticity appears to be a general concept in other nanoribbon systems as well.
authors.affiliation: Institute of Molecular Science, Shanxi University, Taiyuan 030006, People's Republic of China. hj.zhai@sxu.edu.cn lisidian@sxu.edu.cn.
authors.forename: Hui
authors.initials: H
authors.lastname: Bai
authors.forename: Qiang
authors.initials: Q
authors.lastname: Chen
authors.forename: Chang-Qing
authors.initials: CQ
authors.lastname: Miao
authors.forename: Yue-Wen
authors.initials: YW
authors.lastname: Mu
authors.forename: Yan-Bo
authors.initials: YB
authors.lastname: Wu
authors.forename: Hai-Gang
authors.initials: HG
authors.lastname: Lu
authors.forename: Hua-Jin
authors.initials: HJ
authors.lastname: Zhai
authors.forename: Si-Dian
authors.initials: SD
authors.lastname: Li